Method and apparatus for locating the fossa ovalis and performing transseptal puncture

ABSTRACT

A method of identifying the fossa ovalis in a patient by positioning one or more electrodes against the tissue of the interatrial septum of the patient and acquiring unipolar and/or bipolar electrograms of the tissue of the interatrial septum while moving the electrodes to a plurality of positions against the tissue of the interatrial septum. The fossa ovalis is identified on the basis of unipolar voltage reduction, signal fractionation, broadened signal, reduced signal slew rate, reduced local myocardial impedance, increased phase angle and/or increased pacing threshold. An apparatus for identifying the fossa ovalis is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/405,849, filed Aug. 24, 2002 which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to methods and apparatus forlocating the fossa ovalis, as well as methods and apparatus forperforming transseptal punctures.

[0004] 2. Description of Related Art

[0005] The transseptal puncture for left atrial and left ventricularcatheterization was initially described simultaneously by Ross and Copein 1959. In the 1960s, Brockenbrough and colleagues modified the designof the transseptal needle and the guiding catheter. In the late 1960sand 70s, its use declined because of the occurrence of complications andbecause of the development of selective coronary arteriography which ledto the refinement of catheterization of the left side of the heart bythe retrograde approach. With the advent of percutaneous balloon mitralvalvuloplasty, antegrade percutaneous aortic valvuloplasty as well ascatheter ablation of arrhythmias arising from the left atrium (orutilizing left sided bypass tracts), the transseptal puncture techniquefor access to the left atrium and ventricle has made a strong comeback.

[0006] The goal of transseptal catheterization is to cross from theright atrium to the left atrium through the fossa ovalis. Mechanicalpuncture of this area with a needle and catheter combination is requiredfor the procedure. Typically, a guidewire is inserted through the rightfemoral vein and advanced to the superior vena cava. A sheath (typicallyabout 66 cm long) is placed over a dilator (typically about 70 cm long)that is advanced over the guidewire into the superior vena cava. Theguidewire is then removed and a 71 cm Brockenbrough needle is advancedup to the dilator tip. The apparatus is dragged down into the rightatrium, along the septum. When the dilator tip is positioned adjacentthe fossa ovalis (some times determined under ultrasound guidance), theneedle is then pushed forward so that it extends past the dilator tip,through the fossa ovalis into the left atrium. The dilator and sheathmay then be pushed through the fossa ovalis over the needle. The dilatorand needle are then removed, leaving the sheath in place in the leftatrium. Thereafter, catheters may be inserted through the sleeve intothe left atrium in order to perform various necessary procedures.

[0007] The danger of the transseptal approach lies in the possibilitythat the needle and catheter will puncture an adjacent structure such asthe aorta, the coronary sinus or the free wall of the atrium. In theCooperative study on Cardiac Catheterization, a 0.2% mortality rate, 6%incidence rate of major complications and 3.4% incidence of seriouscomplications were reported including 43 perforations. To minimize thisrisk, the operator must have a detailed familiarity with the regionalanatomy of the atrial septum. Due to the potentially life threateningcomplications of the procedure, many operators feel that fluoroscopy,which at best represents an epicardial shadow of the heart, is notenough and additional tools are needed. Additional techniques which maybe used to locate the fossa ovalis include biplane fluoroscopy, pressuremanometry, contrast infusion as well as surface, transesophageal orintracardiac echocardiography (i.e., ultrasound).

[0008] While echocardiography can be useful, there are potentialproblems in using these techniques to locate the fossa ovalis. Contactof the transseptal dilator and the tenting of the membrane of the fossaovalis that one looks for with echo guidance may be missed depending onthe area of the fossa that is cut by the ultrasound beam. If a differentportion of the membrane is tented by the dilator tip, this may not beapparent in the ultrasound picture. If transesophageal echocardiography(TEE) is used to guide the puncture, a different operator has to operatethe TEE system and therefore errors can occur especially in theinterpretation of the data. For example a different catheter (other thanthe transseptal dilator) may be tenting the fossa. Cardiac tamponade andother complications are known to have occurred during transseptalpunctures performed in electrophysiology laboratories despite theroutine use of ultrasound guidance. The placement and use of ultrasoundcatheters also often require the insertion of large intravascularsheaths. The additional time and expense that the use of ultrasoundcatheters and sheaths incurs is not inconsiderable and this can make itimpractical to use them routinely.

SUMMARY OF THE INVENTION

[0009] The present invention provides a method of identifying the fossaovalis in a patient, comprising the steps of:

[0010] (a) positioning one or more electrodes against the tissue of theinteratrial septum of the patient;

[0011] (b) acquiring unipolar and/or bipolar electrograms of the tissueof the interatrial septum, while moving the electrodes to a plurality ofpositions against the tissue of the interatrial septum; and

[0012] (c) identifying the fossa ovalis on the basis of at least one ofthe following parameters:

[0013] unipolar voltage reduction

[0014] signal fractionation

[0015] broadened signal

[0016] reduced signal slew rate

[0017] reduced local myocardial impedance

[0018] increased phase angle and

[0019] increased pacing threshold.

[0020] The fossa ovalis may also be identified on the basis of bipolarvoltage reduction. In addition, the fossa ovalis may be identified onthe basis of at least two of the parameters noted above.

[0021] The present invention also provides a method of performing atransseptal puncture on a patient, comprising the steps of:

[0022] (a) positioning one or more electrodes against the tissue of theinteratrial septum of the patient;

[0023] (b) acquiring unipolar and/or bipolar electrograms of the tissueof the interatrial septum, while moving the electrodes to a plurality ofpositions against the tissue of the interatrial septum;

[0024] (c) identifying the fossa ovalis on the basis of at least one ofthe following parameters:

[0025] unipolar voltage reduction

[0026] signal fractionation

[0027] broadened signal

[0028] reduced signal slew rate

[0029] reduced local myocardial impedance

[0030] increased phase angle and

[0031] increased pacing threshold and

[0032] (d) penetrating the interatrial septum through the fossa ovalisin order to access the left atrium.

[0033] In the above method, the one or more electrodes may be providedon the distal end of a catheter and the positioning step may comprisepositioning the distal end of the catheter against the tissue of theinteratrial septum of the patient. In addition, the penetrating step maycomprise urging a needle through the interior of the catheter andthrough the fossa ovalis into the left atrium. This method may alsoinclude the step of observing ST segment elevation in the unipolarelectrogram in order to ensure that the distal end of the catheter is incontact with the tissue of the interatrial septum.

[0034] The present invention also provides a catheter for use intransseptal punctures, comprising:

[0035] (a) a hollow lumen having a distal end;

[0036] (b) a first electrode positioned at the distal end; and

[0037] (c) a second electrode positioned on the catheter and spacedproximally from the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a light microscopy tissue section from a human atrialseptum;

[0039]FIG. 2 is depicts the use of a quadripolar EP catheter to obtainbipolar electrograms and identify the fossa ovalis on the basis of adecrease in amplitude as the catheter is dragged inferiorly along theinteratrial septum and makes contact with the fossa ovalis;

[0040]FIG. 3 depicts unipolar and bipolar electrograms taken from tissueof the interatrial septum adjacent the fossa ovalis and the fossaitself, with the fossa providing low voltage, broad and fractionatedsignals with low slew rates;

[0041]FIG. 4 depicts unipolar and bipolar electrograms taken from tissueof the interatrial septum adjacent the fossa ovalis, wherein theunipolar electrogram exhibits ST segment elevation while the bipolarelectrogram does not, and the bipolar electrogram exhibits a low voltageeven though the probe was not at the fossa ovalis;

[0042]FIG. 5 is a schematic illustration of the significance of thepositioning of the interelectrode axis in bipolar electrograms withrespect to the orientation of the wavefront of electrical excitation;

[0043]FIG. 6 is similar to FIG. 4, however the location of the probe isshown to indicate that the bipolar electrogram exhibits reduced voltageeven though the probe is not at the fossa ovalis;

[0044]FIG. 7 is a schematic illustration of a transseptal apparatusaccording to one embodiment of the present invention;

[0045]FIG. 8 is a cross-sectional view of the distal portion of thecatheter of the transseptal apparatus shown in FIG. 7;

[0046]FIG. 9 is a schematic illustration of the use of the transseptalapparatus of FIG. 7 for obtaining unipolar electrograms, using a skinpatch as the indifferent (or reference) electrode;

[0047]FIG. 10 is a schematic illustration of the use of the transseptalapparatus of FIG. 7 for obtaining unipolar electrograms, using aWilson's central terminal as the indifferent electrode; and

[0048]FIG. 11 is a schematic illustration of the use of the transseptalapparatus of FIG. 7 for obtaining unipolar electrograms, using separateconventional EP catheter place in the inferior vena cava as theindifferent electrode.

DETAILED DESCRIPTION

[0049] Applicant has discovered that the fossa ovalis can be located bymeasuring electrophysiological activity of the tissue of the interatrialseptum. In addition, Applicant has also developed an apparatus which maybe used not only for locating the fossa ovalis, but also for performingtransseptal punctures through the fossa ovalis.

[0050] The fossa ovalis is the depression at the site of the fetalinteratrial communication termed the foramen ovale. In fetal life, thiscommunication allows richly oxygenated IVC blood coming from theplacenta to reach the left atrium and has a well-marked rim or limbussuperiorly. The floor of the fossa ovalis is a thin, fibromuscularpartition. In fact, because the fossa ovalis is thinner than the rest ofthe interatrial septum, light may even be used to selectivelytransilluminate the fossa ovalis.

[0051] However, in addition to being thinner than the surrounding tissueof the interatrial septum, Applicant has discovered that the fossaovalis has considerable scar tissue. Autopsy studies done on humanhearts (with Masson trichrome stain and microscopy/morphometric analysiswith NIH image program) revealed that the fossa ovalis has significantlymore fibrous tissue that the non fossa portion of the interatrialseptum. Specifically, the non fossa portion of the interatrial septumconsistently had around 70% muscle and 30% fibrous tissue, whereas thefossa ovalis had an average of 33% muscle and 67% fibrous tissue.

[0052]FIG. 1 is a light microscopy tissue section from a human atrialseptum with hematoxylin & eosin staining (panels A, C & E) as well asMasson trichrome stain (panels B, D & F). Panels A & B show the superiorlimbus as well as the membrane of the fossa ovalis. Panels C & D showthe superior limbus of the interatrial septum, while panels E & F showthe membrane of the fossa ovalis. As noted from FIG. 1, the fossa ovalishas significantly more fibrous scar tissue than other portions of theinteratrial septum. Applicant has found that this increased scar tissue,coupled with the reduced thickness of the fossa ovalis, allows one tolocate the fossa by measuring electrophysiological activity of theregion.

[0053] In particular, Applicant has found that the fossa ovalis may belocated by measuring the electrophysiological (“EP”) activity of thefossa ovalis and surrounding heart tissue. By observing differences inthe EP activity of tissue at various locations, the operator maydetermine the location of the fossa ovalis. The lower muscle content andhigher fibrous tissue content of the fossa ovalis with respect to therest of the interatrial septum, as well as the relative “thinning” ofthe fossa, results in changes in EP activity which may be readilyobserved via an intracardiac electrogram. For example, the fossa ovaliswill record broader, fractionated electrograms of lower amplitude andlower slew rates. Based upon these surprising findings, one or moreelectrodes for acquiring EP data may be incorporated into acatheter/dilator used during transseptal puncture.

[0054] Intracardiac electrograms are typically performed by positioninga probe having one or more electrodes against the cardiac tissue to beexamined. The probe is typically inserted into the heart through a veinor artery via a sheath. A recording device (well-known to those skilledin the art) is used to record and display an electrical signal producedby the cardiac tissue. When a unipolar probe is used, the lead from thesingle electrode on the probe is connected to the positive terminal of arecording device or amplifier, and an indifferent (or reference)electrode is then connected to the negative terminal of the recordingdevice or amplifier. The indifferent electrode is simply anotherelectrode which is located away from the “exploring” electrode—typicallypositioned within or against a structure in which no electrical activitytakes place. The reference electrode may comprise a skin patch, aplurality of electrodes placed at various locations on the patient'sbody (i.e., Wilson's central terminal), or even an electrode placed in aseparate catheter positioned in a large vein (e.g., the inferior venacava). For unipolar electrograms, the electrogram will display thedifference in electrical potential between the unipolar electrodepositioned against the cardiac tissue and the electrode connected to thenegative electrode.

[0055] For bipolar electrograms, the probe typically comprises twoelectrodes spaced apart from one another by a short distance (typicallyless than about 5 mm) in a single probe. The leads from the twoelectrodes are connected to a junction box of a recording device.

[0056] A signal in an intracardiac electrogram is characterized in termsof its amplitude (typically measured in millivolts), its duration(measured in milliseconds) and its slew rate (measured in volts persecond). The amplitude of an electrogram depends on several factors,including the mass of myocardium underlying the electrode, the contactof the electrode with the myocardium, the orientation of the electrodewith respect to the axis of depolarization, and the presence of anyinexcitable tissue (i.e., fibrous or connective tissue) between themyocytes and the electrode. Electrodes that are in contact withinfarcted ventricular myocardium or those that have become encapsulatedwith a thick layer of fibrous tissue, for example, typically show alower amplitude electrogram than those directly in contact with healthymyocytes. The slew rate represents the maximal rate of change of theelectrical signal between the sensing electrodes and, mathematically, isthe first derivative of the electrogram (dv/dt) and is a measure of thechange in electrogram voltage over time. It is generally directlyrelated to the electrogram amplitude and duration. In the region of thefossa ovalis, the slew rate will be lower in the region of the fossaovalis, thus following the changes in electrogram amplitude.

[0057]FIG. 2 depicts a bipolar electrogram acquired using a standard EPdeflectable catheter 30. In FIG. 2, quadripolar EP catheter 30 islocated within the right atrium, against the interatrial septum abovethe fossa ovalis 25 (see FIG. 2A). For point of reference, the superiorvena cava is depicted at 21, the aorta at 22, the pulmonary artery at 23and the coronary sinus at 24. As the catheter 30 is dragged inferiorlyalong the interatrial septum and makes contact with the fossa ovalis 25(FIG. 2B), the bipolar electroanatomical voltage 31 drops abruptly. Infact, the region of the fossa ovalis consistently displays unipolar andbipolar electrograms of amplitudes less than about 1 mV while the restof the interatrial septum displays electrograms greater than about 2 mV(see FIG. 3).

[0058] High density mapping of the interatrial septum (performed withthe CARTO system available from Biosense Webster, Diamond Bar, Calif.)further revealed that the area of the fossa ovalis (posterior to thecoronary sinus ostium and inferior to the His bundle) consistently showslow voltage and fractionated unipolar and bipolar electrograms. FIG. 3depicts bipolar and unipolar electrograms obtained using the CARTOsystem. Panel A of FIG. 3 depicts bipolar and unipolar electrograms fora location on the interatrial septum superior to the fossa ovalis. Asnoted in panel A of FIG. 3, the unipolar voltage is 2.66 mV and thebipolar voltage is 2.52 mV. Panel B of FIG. 3 depicts bipolar andunipolar electrograms for the fossa ovalis 25. As noted in panel B ofFIG. 3, the unipolar voltage for the fossa ovalis is 0.52 mV and thebipolar voltage is 0.84 mV. Thus, the drop in voltage for the fossaovalis as compared to the surrounding tissue of the interatrial septummay be effectively used to locate the fossa ovalis.

[0059] As seen in FIG. 3, the region of the fossa ovalis also displayselectrograms that are broad (greater than about 50 ms in duration) whilethe rest of the interatrial septum has signals that are less than about35 ms in width. Bipolar electrograms also show multiple spikes ordeflections (fractionation), and the unipolar electrogram is broad witha low slew rate. This fractionation in the electrogram morphology isthought to be due to the substantially greater amounts of collagen inthis tissue compared to the rest of the atrium resulting in complexanisotropic conduction. With respect to the slew rate in unipolarelectrograms, the slew rate observed for the fossa ovalis is less thanor equal to about 0.3 volts/second, whereas the rest of the interatrialseptum provides slew rates of greater than about 0.5 volts/second. Thus,the increased fractionation, increased signal width and decreased slewrate provide additional indicia of the location of the fossa ovalis.

[0060] Not only can the fossa ovalis be identified by observing reducedsignal amplitude, and increased width, fractionation and slew rate, thefossa will also be evidenced by reduced local myocardial impedance andhigher phase angle. Myocardial tissue impedance is considered to be apassive electrical property of both healthy and diseased tissues. Themyocardial impedance (Z) is defined as the voltage (V) divided by thesinusoidal current (I) applied through it. It has been noted by severalinvestigators that ventricular aneurysms from chronic myocardialinfarctions display a lower local myocardial impedance measurement and ahigher phase angle. Changes in impedance measurements were found to bereliable in identifying scar tissue and in defining the presence, extentand location of chronic myocardial infarction. The lower endocardialimpedance values in scar tissues is thought to be due to an increase inthe extracellular-to-intracellular volume ratio and a resultant increasein the extracellular pathways of impulse conduction. Another hypothesisis that the improved conductance may be due to the thinning of the leftventricular wall and loss of cardiac tissue mass.

[0061] Applicant has found that the lower muscle content and higherfibrous tissue content of the fossa with respect to the rest of theinteratrial septum as well as the relative “thinning” of the fossaovalis will result in the recording of lower local impedance values andthus can be used to identify the fossa ovalis. When the catheter ispositioned in the superior vena cava, the system generally recordedimpedance values of greater than about 130 ohms. Once the catheterdescended into the right atrium, the impedance decreased to about 120ohms or less. Dragging the catheter to the region of the fossa ovalislowered the impedance value by about another 15 ohms. Therefore, byobserving the myocardial impedance as the tip of the catheter or otherprobe is dragged inferiorly across the interatrial septum, the fossaovalis also can be identified by a sharp drop in impedance (typicallyabout 15 ohms). Similarly, the fossa ovalis can also be identified by anincrease in phase angle as compared to the surrounding tissue.

[0062] Impedance may be measured, for example, using the techniquedescribed by Wolf et al, Am. J. Physiol. 2001; 280: H179-H188. Agenerator (such as a Stockert generator, available from BiosenseWebster, Diamond Bar, Calif.) with a stabilized output amplitude, andproducing a sine wave signal of 1-2 uA with a frequency of 50 kHz, isused. The output current source buffer with a high output impedance isconnected to an intracardiac electrode and provided constant alternatingcurrent through the myocardial tissues. A return electrode is connectedto the reference point in the circuit and placed against the patient'sback. The intracardiac electrode is connected to one input of adifferential amplifier and the return electrode is connected to thesecond input. The output of the amplifier is then passed through a bandpass filter with a central frequency equal to the measuring frequency. Asynchronous detector converts the AC voltage to the direct current.

[0063] The presence of greater amounts of fibrous tissue in the fossaovalis will also result in a higher pacing threshold and thus, willserve as an additional electrophysiological measurement in identifyingthe fossa ovalis. “Pacing threshold” refers to the amount of currentfrom a pacemaker electrical impulse required to capture the heart. It isheavily dependent on the amount of muscle and scar/fibrous tissue thatthe pacing catheter is in contact with. The more scar/fibrous tissuethat is present, the higher the pacing threshold.

[0064] Thus, an electrical pacing impulse may be applied to theinteratrial septum through an electrode (e.g., a standard EP deflectablecatheter or even an electrode on the transseptal apparatus of thepresent invention described further herein). The minimum amplituderequired to capture the heart is applied and the probe or cathetercarrying the electrode is dragged inferiorly across the interatrialseptum. As the electrode reached the fossa ovalis, the electricalimpulse will no longer be of sufficient amplitude to capture the heart.For example, an electrical impulse having a pulse width duration ofabout 0.5 msec and an amplitude of between about 0.8 and about 1.0 Vwill generally be sufficient to capture the hear when applied to theinteratrial septum adjacent the fossa ovalis. However, this amplitudewill not be sufficient to capture the heart when applied to the fossaovalis. Here, an amplitude of between about 1.5 and about 1.6 V isrequired, at a pulse width duration of 0.5 msec. Thus, the increasedpacing threshold may also be used to locate the fossa ovalis.

[0065] In addition the various parameters described above for locatingthe fossa ovalis, the ST segment elevation observed in unipolarelectrograms can also be advantageously employed during transseptalpuncture. The unipolar electrogram of the tissue of the interatrialseptum will display ST segment elevation whenever the electrode makesgood tissue contact with, and exerts significant pressure against theatrial myocardium. As seen in FIG. 4, ST segment elevation is recordedin the unipolar electrogram of the interatrial septum but not thebipolar electrogram. By observing ST segment elevation, the operator canbe confident that the catheter is exerting significant pressure on themyocardium for transseptal puncture (via a needle passed through thecatheter).

[0066] The above mentioned electrophysiological parameters can be usedto guide the transseptal puncture procedure. For example, a singleelectrode may be incorporated into the tip of the dilator of thetransseptal apparatus (to record unipolar electrograms and otherelectrophysiological parameters). Alternatively, a pair of electrodesmay be incorporated into the dilator in order to measure bipolarelectrograms. Because there are several electrophysiologicaldistinctions between the fossa ovalis and the rest of the atrialmyocardium, the use of multiple electrophysiological parameters will addto the predictive value of the measurements made by the apparatus.

[0067] Although bipolar electrograms can be useful in locating the fossaovalis, bipolar electrograms have a serious flaw in that if thewavefront of electrical activity travels in a direction perpendicular tothe interelectrode axis, no electrical activity may be recorded. Giventhe potential for life threatening complications if a transseptalpuncture were to be made in the wrong area of the atrium, this canliterally be a fatal flaw if one were to rely solely on voltagemeasurements taken from bipolar electrograms. The electrophysiologicalbasis for this flaw and other advantages of unipolar electrograms arediscussed below.

[0068] Potentials generated by current sources in a volume conductorsuch as the heart are always recorded with respect to the potential at areference site. Thus, it is the difference in electrical potentialbetween the two electrodes produced by electrical currents within themyocardium that generates the intracardiac electrogram. In practice, thesignal at the recording site is fed into the positive input and thereference signal into the negative input of a recording device.

[0069] A bipolar electrogram can be considered to be the instantaneousdifference in potential between two unipolar electrograms recorded fromtwo unipolar electrodes and a common remote indifferent electrode. Inmathematical terms, the bipolar electrogram is equal to the unipolarelectrogram from first electrode 35 minus the unipolar electrogram fromsecond electrode 36. In other words:

BiEGM=UniEGM1−UniEGM2.

[0070] If the unipolar tip and ring electrograms have similar amplitudeand timing, the two signals will cancel each other out and the resultingbipolar electrogram will be nonexistent or much smaller than eitherunipolar electrogram alone.

[0071] Because of this, bipolar electrograms are susceptible to theorientation of the interelectrode axis with respect to the depolarizingwavefront. As shown in panels A and B of FIG. 5, if the axis of theelongate probe (i.e., the interelectrode axis) is parallel to thedirection in which the depolarizing wavefront is advancing, a sharpelectrogram will be inscribed. However, if the interelectrode axis isperpendicular to the wavefront (panels C and D of FIG. 5), as thewavefront passes underneath them, the shift in potential beneath the twoelectrodes will be identical. The electrodes in this scenario, willrecord no difference in electrical potential with respect to each other.Consequently, no intracardiac electrogram is generated. In general, itis recognized that bipolar electrograms are a function of threevariables: the voltage of the tip or distal electrode, the voltage ofthe ring or proximal electrode, and the presence of activation timedifference (phase shift) between the poles or electrode. This is incontrast with unipolar electrograms where the only variable is thevoltage of the tip electrode. Due to the increased number of variables,the greater variance in bipolar electrograms is not surprising. Thelarge signal variation associated with normal respiration often seenwith bipolar electrograms is additional evidence for theirinconsistency.

[0072] A dramatic illustration of the potential problems associated withrelying on bipolar electrograms alone is shown in FIG. 6 which depicts afalse reading of low voltage by bipolar electrogram. Here, a catheter 34having first and second electrodes 35 and 36 is positioned such that thecatheter tip (and hence both electrodes) are positioned in the posteriorwall of a human atrium away from the fossa ovalis. A transseptalpuncture performed at this site can have life threatening complications.However, the bipolar electrogram from this site indicates a voltage ofonly 0.95 mV, while the unipolar electrogram exhibits a voltage of 2.43mV (indicating that the tip of the catheter is not at the fossa ovalis).This discrepancy is more than likely the result of unipolar EGMsrecorded by the distal and proximal electrodes of the catheter being ofsimilar amplitude and that there is no significant phase shift in timingof activation. Thus, the voltage of the bipolar electrogram signalshould not be relied upon, by itself, to locate the fossa ovalis.

[0073] The principles and sensing configuration described above, i.e.recording of electrical signals from a pair of electrodes both of whichare in contact with the myocardium, describe the bipolar system. In thebipolar recording mode, the reference electrode is positioned close tothe exploring electrode. In the unipolar mode, the reference electrode(usually the anode) is located at an infinite distance from therecording site (cathode). Theoretically, the reference electrode islocated beyond the zone of current flow that generates the electricalfield at the exploring electrode. Therefore, a unipolar recording mayreflect influences from both local and distant electrical events.Unipolar electrograms recorded from the heart may include activity fromother parts of the heart, although their contribution decreases withdistance. Simultaneous recordings of intra- and extracellularelectrograms have shown that for the downstroke of the unipolarelectrogram, the intrinsic deflection coincides with the upstroke of theaction potential beneath the exploring electrode.

[0074] In clinical practice, during the recording of unipolarelectrograms with pacemakers, the pulse generator functions as thereference electrode. In the electrophysiology laboratory, the referenceelectrode is usually one of the following:

[0075] a) Skin patch: A wire is connected to the skin patch and leads tothe negative terminal of the amplifier. During implantation of unipolarpacemaker leads, the lead is often tested with an alligator clipconnected to muscle in the exposed pocket and the other end connected tothe negative terminal of the amplifier (using the same principle as theskin patch).

[0076] b) Wilson's central terminal: Central terminal created byconnecting all three limb electrodes through a 5000 ohm resistor. Thisterminal is used as the negative pole.

[0077] c) Indifferent electrode in one of the great veins such as theinferior vena cava: A separate catheter that is connected to thenegative input of the amplifier may also be placed in one of the greatveins.

[0078] Investigators have found that the peak to peak amplitude of theunipolar signal correlated quantitatively with local histologic ischemicchanges in the setting of ischemic cardiac injury. In patients withcardiac scars and aneurysms from prior myocardial infarctions, voltagemapping with unipolar electrograms reliably differentiates normallyperfused myocardium from fixed perfusion defects, and also identifiesnon-viable zones within the perfusion defects. In the study by Keck etal, the unipolar voltage findings correlated well with findings fromPositron Emission Tomography (PET) glucose utilization. Bipolarelectrograms are often difficult to interpret especially when thesignals are fractionated (when they have multiple deflections). Incontrast, the interpretation of the unipolar electrogram isstraightforward, even under abnormal conditions and this feature is itsgreat strength.

[0079]FIGS. 7 and 8 depict a transseptal apparatus 50 according to thepresent invention which may be used to not only locate the fossa ovalisbut also to perform a transseptal puncture. Transseptal apparatus 50 issimilar to conventional transseptal apparatus in that it includes ahollow sheath 51 and an internal catheter (sometimes referred to as adilator) 52. Catheter 52 is hollow and is slightly longer than sheath 51(typically about 4 cm longer). As is known to those skilled in the art,a guidewire is inserted through the right femoral vein and advanced tothe superior vena cava. Catheter (or dilator) 52 is inserted into sheath51, with the distal end of the catheter protruding beyond the distal end56 of sheath 51. The sheath and catheter are then advanced over theguidewire into the superior vena cava. The guidewire is then removed.

[0080] Not only is the distal end 70 of catheter 52 tapered in theconventional manner, a pair of electrodes 65 and 66 are provided at thedistal end of catheter 52. First, or distal, electrode 65 is provided atthe tip of catheter 52, and second, or proximal, electrode 66 may alsobe provided at the distal end of catheter 52, spaced proximally fromfirst electrode 65 by a distance of between about 2 and about 4 mm. Theelectrodes may, for example, be ring-shaped, with the first electrodemeasuring between about 2 mm and about 4 mm in length, and the secondelectrode measuring about 2 mm in length. Electrical leads 73 and 74 arein electrical communication with first and second electrodes 65 and 66,respectively. At the proximal end of catheter 52, electrical leads 73and 74 are in electrical communication with cables 53 and 54,respectively, which may be attached to a differential amplifier or otherdevice for generating electrograms. In this manner, the tip portion 70of catheter 52 will function as an electrophysiology mapping catheter(both bipolar and unipolar), and will also serve the same function as acatheter/dilator in a conventional transseptal apparatus.

[0081] FIGS. 9-11 depict the use of transseptal apparatus 70 in atransseptal puncture using unipolar electrograms to locate the fossaovalis. Since only unipolar measurements are depicted, the secondelectrode has been omitted from the tip 70 of catheter 52. However, itshould be noted that, even if second electrode 66 is provided oncatheter 52, the catheter can still be used for unipolar (as well asbipolar) measurements. In FIG. 9, transseptal apparatus 50 has beeninserted into the patient's femoral vein until the distal tip 70 of thecatheter/dilator is located within superior vena cava. The electricallead from electrode 65 on tip 70 is used as the positive input to adifferential amplifier, while a skin patch 75 serves as the referenceelectrode and is attached to the negative input to the differentialamplifier via a wire or other lead. Distal tip 70 having first electrode65 is then dragged inferiorly along the interatrial septum while theoperator monitors the electrophysiological parameters for indicationthat the distal tip 70 has made contact with the fossa ovalis. One ormore of the indicators described previously may be employed to make thisdetermination. In addition, the operator may also observe ST segmentelevation in order to confirm that he distal tip 70 is exertingsignificant pressure against the fossa ovalis.

[0082] Once the operator has confirmed the location of the fossa ovalisand that the distal tip 70 is in good contact with the fossa ovalis, aneedle may be urged through the central lumen of catheter 52 until thetip of the needle protrudes beyond distal tip 70 through the fossaovalis and into the left atrium. Thereafter, the catheter 52 may beurged through the fossa ovalis, followed by sheath 51. The catheter 52and needle are then removed from sheath 51, leaving sheath 51 extendingthrough the fossa ovalis into the left atrium.

[0083]FIG. 10 depicts an alternative arrangement wherein the Wilson'scentral terminal is used as the reference electrode. Wilson's centralterminal is created by connecting all three limb electrodes through a5000 ohm resistor. This terminal is used as the negative pole.

[0084]FIG. 11 depicts yet another alternative arrangement wherein anindifferent electrode 76 is positioned in the inferior vena cava. Aseparate catheter that is connected to the negative input of theamplifier may also be placed in one of the great veins.

I claim:
 1. A method of identifying the fossa ovalis in a patient,comprising the steps of: (a) positioning one or more electrodes againstthe tissue of the interatrial septum of the patient; (b) acquiringunipolar and/or bipolar electrograms of the tissue of the interatrialseptum, while moving said electrodes to a plurality of positions againstsaid tissue of the interatrial septum; and (c) identifying the fossaovalis on the basis of at least one of the following parameters:unipolar voltage reduction signal fractionation broadened signal reducedsignal slew rate reduced local myocardial impedance increased phaseangle and increased pacing threshold.
 2. The method of claim 1, whereinthe fossa ovalis is also identified on the basis of bipolar voltagereduction.
 3. The method of claim 1, wherein the fossa ovalis isidentified on the basis at least two of the following parameters:unipolar voltage reduction signal fractionation broadened signal reducedsignal slew rate reduced local myocardial impedance increased phaseangle and increased pacing threshold.
 4. A method of performing atransseptal puncture on a patient, comprising the steps of: (a)positioning one or more electrodes against the tissue of the interatrialseptum of the patient; (b) acquiring unipolar and/or bipolarelectrograms of the tissue of the interatrial septum, while moving saidelectrodes to a plurality of positions against said tissue of theinteratrial septum; (c) identifying the fossa ovalis on the basis of atleast one of the following parameters: unipolar voltage reduction signalfractionation broadened signal reduced signal slew rate reduced localmyocardial impedance increased phase angle and increased pacingthreshold and (d) penetrating the interatrial septum through the fossaovalis in order to access the left atrium.
 5. The method of claim 4,wherein said one or more electrodes are provided on the distal end of acatheter and said positioning step comprises positioning the distal endof said catheter against the tissue of the interatrial septum of thepatient.
 6. The method of claim 5, wherein said penetrating stepcomprises urging a needle through the interior of said catheter andthrough the fossa ovalis into the left atrium.
 7. The method of claim 5,wherein two electrodes are provided on said catheter, one of saidelectrodes at the distal end of the catheter and the other of saidelectrodes is located on said catheter proximal to the other electrode.8. The method of claim 5, wherein a bipolar electrogram is acquired andfurther comprising the step of observing ST segment elevation in theunipolar electrogram in order to ensure that the distal end of saidcatheter is in contact with the tissue of the interatrial septum.
 9. Themethod of claim 4, wherein the fossa ovalis is also identified on thebasis of bipolar voltage reduction.
 10. The method of claim 4, whereinthe fossa ovalis is identified on the basis at least two of thefollowing parameters: unipolar voltage reduction signal fractionationbroadened signal reduced signal slew rate reduced local myocardialimpedance increased phase angle and increased pacing threshold
 11. Acatheter for use in transseptal punctures, comprising: (a) a hollowlumen having a distal end; (b) a first electrode positioned at saiddistal end; and (c) a second electrode positioned on said catheter andspaced proximally from said first electrode.